This invention relates generally to teletext receiving systems. It is particularly directed to a receiving system capable of indicating to a user the receipt of a valid teletext signal.
Teletext is a generic term for a television based communication technique which uses the vertical blanking interval of a broadcast television signal for transmission of text and graphics information. Systems implementing this technique are presently operational in various countries, such as in England where teletext formatted text and graphics material is transmitted in digitally encoded form on lines 17, 18, 330 and 331 of their 625 line system. Television receivers equipped with teletext decoders process the transmitted data for forming a display of the text and graphics on their viewing screens.
In normal operation, a television receiver which is equipped to receive and display teletext information will generate a display of the teletext information only upon command by the television viewer. Lacking such a command, the television receiver displays the video associated with the conventional television broadcast signal. Any teletext information being simultaneously broadcast is not visible to the viewer, as that information is encoded in the vertical blanking interval of the television signal which is blanked from the viewing screen. Hence, while the television receiver is displaying a conventional broadcast signal, the viewer is ordinarily unaware that the teletext signal is being simultaneously received. The viewer, therefore, may miss a teletext transmission which he wishes to view, and/or record on a video tape recorder or the like.
Accordingly, it is a general object of the invention to provide an automatic indicator which indicates the receipt of a teletext transmission.
It is another object of the invention to provide such an indicator which is responsive only to a valid teletext signal and which is unresponsive to spurious signals which may intermittently appear as a teletext signal.
It is a further object of the invention to provide a control signal which responds to a valid teletext signal for automatically actuating the recording thereof.